Structural resins with a favorable combination of properties such as long term environmental durability over a temperature range of -54.degree. to 220.degree. C., damage tolerance, and solvent resistance are required for use in hot areas on military and commercial aircraft. Commercially available bismaleimide resins such as Compimide 353 (available from Boots/Technochemie) and XU-292 (available from Ciba/Geigy Corporation) previously have been formulated with other components to enhance properties such as melt processability and toughness. These formulated systems have found use in a variety of applications such as composite matrices and adhesives for use at temperatures in excess of those at which epoxy systems can perform.
Polyaspartimides are commonly synthesized by reaction of aromatic bismaleimides with aromatic diamines. These polymers have a repeat unit of the following general formula: ##STR1## where Ar and Ar' are divalent aromatic radicals, such as 1,3-phenylene or 4,4'-oxydiphenylene.
High molecular weight polyaspartimides are known [J. V. Crivello, J. Polymer Sci., 11, 1185(1973)] and can form tough, flexible films with good tensile properties and can be compression molded. However, uncrosslinked polyaspartimides are susceptible to solvent attack, especially in a stressed condition, and upon exposure undergo solvent induced stress crazing and cracking. In addition, polyaspartimides are generally soluble in dipolar solvents such as N,N-dimethylformamide, dimethyl sulfoxide, N,N-dimethylacetamide, N-methylpyrrolidinone, phenol, and m-cresol. Melt condensation of aromatic diamines and bismaleimides has been reported to yield cross-linked polymers (polyaspartimides) [F. Grundschober and J. Sambeth (To Soc. Rhodiacita), U.S. Pat. No. 3,533,996 (Oct. 13, 1970)].
In most cases, less than a stoichiometric amount of an aromatic diamine is reacted with a bismaleimide to yield a low or intermediate molecular weight polyaspartimide terminated with maleimide groups. Upon heating, the terminal maleimide groups react to yield a crosslinked resin having less toughness than the linear polyaspartimides but with improved solvent resistance and higher use temperature. The degree of crosslinking can be controlled to a large extent by the stoichiometry. A common material of this type is Kerimid 601 (available from Rhone Poulenc), used extensively in printed circuit boards.
The acetylene terminated aspartimides (ATA) of this invention present new compositions of matter. The properties of cured acetylene terminated aspartimides are similar to those of the cured unformulated bismaleimides but they have significantly higher toughness. In addition, the acetylene terminated aspartimides are formed as stereoisomers (d, l and meso isomers), and as such have lower melt temperatures than bismaleimides with similar chemical structure. Since they are a mixture of isomers, they also melt over a broad temperature range. Both of these factors make acetylene terminated aspartimides more amenable to process into adhesive tape and prepreg via melt technique than comparable bismaleimides.
The acetylene terminated aspartimides can be readily blended with acetylene terminated arylene ether oligomers and polymers to yield cured resins with high toughness. The crosslink density can be controlled by varying the ratio of acetylene terminated aspartimide to acetylene terminated arylene ether oligomers or polymers and also by varying the length and chemical structure of the linear segment in the acetylene terminated arylene ether oligomers or polymers. The acetylene terminated aspartimides can also be blended with acetylene terminated polysulfones to yield resins with high fracture toughness.
Accordingly, it is an object of this invention to provide a method of preparing acetylene terminated aspartimides.
A further object of this invention is to provide acetylene terminated aspartimides that are useful as adhesives, coatings, films and composite matrices.
A further object of the present invention is to provide blends of acetylene terminated aspartimides and other acetylene terminated oligomers and polymers that are useful as adhesives, coatings, films, membranes and composite matrices.